Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (VDF) (so called THV copolymers) and polyvinylidene fluoride polymers (PVDF). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers.
Several methods are known to produce fluoropolymers. Such methods include suspension polymerization, aqueous emulsion polymerization, solution polymerization, polymerization using supercritical CO2, and polymerization in the gas phase.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used for the stabilization of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally good stability over a long period of time is obtained.
It is generally recognized that an aqueous emulsion polymerization wherein no surfactant is used to generally produce homo- and copolymers of chlorotrifluoroethylene (CTFE).
Notwithstanding the fact that emulsifier free polymerizations are known, the aqueous emulsion polymerization process in the presence of fluorinated surfactants is still a desirable process to produce fluoropolymers because it can yield stable fluoropolymer particle dispersions in high yield and in a more environmental friendly way than for example polymerizations conducted in an organic solvent. Frequently, the emulsion polymerization process is carried out using a perfluoroalkanoic acid or salt thereof as a surfactant. These surfactants are typically used as they provide a wide variety of desirable properties such as high speed of polymerization, good copolymerization properties of fluorinated olefins with comonomers, small particle sizes of the resulting dispersion can be achieved, good polymerization yields i.e. a high amount of solids can be produced, good dispersion stability, etc. However, environmental concerns have been raised against these surfactants and moreover these surfactants are generally expensive. Alternative surfactants to the perfluoroalkanoic acids or salts thereof have also been proposed in the art for conducting the emulsion polymerization of fluorinated monomers.
It would now be desirable to find an alternative emulsion polymerization process in which the use of perfluoroalkanoic acids and salts thereof as a fluorinated surfactant can be avoided. In particular, it would be desirable to find an alternative surfactant or dispersant, in particular one that is more environmentally friendly, for example has a low toxicity and/or shows no or only little bioaccumulation. It would also be desirable that the alternative surfactant has good chemical and thermal stability enabling polymerization over a wide range of conditions of for example temperature and/or pressure. Desirably, the alternative surfactant or dispersant allows for a high polymerization rate, good dispersion stability, good yields, good copolymerization properties; less or no telogenic effects and/or the possibility of obtaining a wide variety of particle sizes including small particle sizes. The properties of the resulting fluoropolymer should generally not be negatively influenced and preferably would be improved. Desirably, the resulting dispersions have good or excellent properties in coating applications and/or impregnation of substrates, including for example good film forming properties. It would further be desirable that the polymerization can be carried out in a convenient and cost effective way, preferably using equipment commonly used in the aqueous emulsion polymerization of fluorinated monomers. Additionally, it may be desirable to recover the alternative surfactant or dispersant from waste water streams and/or to remove or recover the surfactant from the dispersion subsequent to the polymerization. Desirably, such recovery can proceed in an easy, convenient and cost effective way.